Method of controlling channel access

ABSTRACT

An embodiment of the present invention provides a method of controlling channel access in a WLAN system, comprising receiving, from an access point, a first channel access control message sent to stations located in some area through a directional antenna of the access point, and attempting to access a channel according to the first channel access control message. The method enables coexistence of several users and improves scalability of a wireless system.

TECHNICAL FIELD

The present invention relates to a Wireless Local Access Network (WLAN),and more particularly to, a method of controlling channel access, when aWLAN system is applied in an environment where it can coexist withanother wireless communication system.

BACKGROUND ART

There are frequency bands in which different kinds of wirelesscommunication systems can coexist. One example of the frequency bands isa TV white space. The TV white space is an idle frequency band reserveddue to digitalization of an analog broadcasting. The TV white space is aspectrum of 512 to 698 MHz allocated to a broadcast TV.

When a licensed device is not used in the corresponding spectrum, aunlicensed device can use the corresponding band. When the unlicenseddevice intends to use the TV white space, it should acquire an availablechannel in a corresponding area, using a geo-location database.

In addition, a signaling protocol such as a common beacon frame isnecessary to solve a problem in coexistence of the unlicensed devicesusing the TV white space.

When IEEE 802.11 is used in the TV white space, there is an advantage inthat the coverage is remarkably expanded due to the spectrum'scharacteristic.

However, generally, as the coverage is expanded, the number of stationsconsiderably increases. In this situation, a problem may arise in acapability of flexibly handling users with increase of the number of theusers, i.e. scalability. Moreover, since several wireless communicationsystems coexist and several unlicensed devices coexist, a problem mayarise in coexistence. If a Distributed Coordination Function (DCF) andan Enhanced Distributed Channel Access (EDCA) protocol of IEEE 802.11are applied in this environment, scalability may be more deteriorated.

The DCF is a channel access mechanism used in IEEE 802.11 and is basedon Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). Also,the EDCA is equivalent to a competition-based medium access method amongchannel access models suggested by a Hybrid Coordination Function (HCF)defined by expanding a general medium access control protocol of IEEE802.11. Here, the HCF is a protocol defined in IEEE 802.11e suggested toensure Quality of Service (QoS).

Further, when the protocol of IEEE 802.11 is applied in the TV whitespace, it is expected that overlapping Basic Service Sets (BSSs) willincrease sharply. For example, if a user arbitrarily installs an AccessPoint (AP) supporting the TV white space, since the coverage of the APis wide, an overlapping service area may increase.

DISCLOSURE OF INVENTION Technical Problem

An embodiment of the present invention is to improve scalability orcoexistence in a competition-based channel access mechanism, andparticularly, to reduce problems in scalability and coexistence, when aWLAN system is used in a frequency band where wireless communicationsystems coexist.

In addition, an embodiment of the present invention is to provide achannel access method which sets priorities differently according to acategory of a data frame, so that it is possible to reduce a delay timeof data sensitive to delay.

Therefore, the present invention is intended to provide stable QoS to aplurality of users.

Technical Solution

According to an aspect of the present invention, there is provided amethod of controlling channel access in a WLAN system, comprisingreceiving, from an access point, a first channel access control messagesent to stations located in some area through a directional antenna ofthe access point, and attempting to access a channel according to thefirst channel access control message.

Advantageous Effects

According to an embodiment of the present invention, scalability orcoexistence can be improved in a competition-based channel accessmechanism. Particularly, scalability and coexistence between users canbe secured in an environment where different kinds of wirelesscommunication systems are applied.

A delay time of data sensitive to delay can be reduced relatively. As aresult, according to an embodiment of the present invention, an improvedQoS can be provided to a plurality of users.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram illustrating configuration of one exampleof a Very High Throughput (VHT) WLAN system to which embodiments of thepresent invention can be applied;

FIG. 2 is a diagram illustrating inter-frame spaces applied to a channelaccess mechanism in the WLAN system;

FIG. 3 is a diagram illustrating a channel access method according toone embodiment of the present invention;

FIG. 4 is a diagram illustrating a channel access control messagetransmitted by the channel access method explained with reference toFIG. 3;

FIG. 5 is a diagram illustrating a channel access method according toanother embodiment of the present invention;

FIG. 6 is a diagram illustrating a channel access control messagetransmitted by the channel access method explained with reference toFIGS. 5; and

FIG. 7 is a diagram illustrating a Traffic Indication Map (TIM) of thechannel access control message explained with reference to FIG. 4 or 6.

MODE FOR THE INVENTION

FIG. 1 is a schematic diagram illustrating configuration of one exampleof a VHT WLAN system to which embodiments of the present invention canbe applied.

Referring to FIG. 1, the WLAN system such as the VHT WLAN systemincludes one or more Basic Service Sets (BSSs). The BSS is a set ofstations (STA) which can be successfully synchronized and communicatewith one another, and is not a concept indicating a specific area. Inaddition, a BSS which supports ultrahigh speed data processing over 1GHz in a Media Access Control (MAC) Service Access Point (SAP) as in theWLAN system to which the embodiments of the present invention can beapplied is called a Very High Throughput (VHT) BSS.

The VHT BSS can be classified into an infrastructure BSS and anindependent BSS (IBSS). The infrastructure BSS is shown in FIG. 1.

The infrastructure BSSs BSS1 and BSS2 include one or more non-AP STAsSTA1,

STA3 and STA4, access points (APs) STA2 and STA5 which are STAsproviding a distribution service, and a distribution system (DS)connecting the plurality of APs STA2 and STA5. In the infrastructureBSS, the AP STA manages the non-AP STAs of the BSS.

On the contrary, the IBSS is a BSS operating in an Ad-Hoc mode. Sincethe IBSS does not include an AP VHT STA, there is no centralizedmanagement entity performing a management function in the center. Thatis, in the IBSS, non-AP STAs are managed in a distributed manner. In theIBSS, all STAs may be mobile stations configuring a self-containednetwork, since access to a DS is disallowed.

The STA is an optional functional medium including Medium Access Control(MAC) and a physical layer interface for a wireless medium in accordancewith an IEEE 802.11 standard, and includes both APs and non-AP STAs in awide meaning. Moreover, an STA which supports ultrahigh speed dataprocessing over 1 GHz in a multi-channel environment described below iscalled a VHT STA. In the VHT WLAN system to which the embodiments of thepresent invention can be applied, the STAs included in the BSS may beVHT STAs, or VHT STAs and legacy STAs (e.g. HT STAs in accordance withIEEE 802.11n) which coexist with one another.

Among the STAs, a handheld terminal manipulated by a user is a non-APSTA STA1, STA3, STA4 and STA5. Simply, an STA may mean a non-AP STA. Thenon-AP STA may be called a terminal, a Wireless Transmit/Receive unit(WTRU), a User Equipment (UE), a Mobile Station (MS), a mobile terminal,a mobile subscriber unit, and so on. Moreover, a non-AP STA whichsupports ultrahigh speed data processing over 1 GHz in a multi-channelenvironment described below is called a non-AP VHT STA or a VHT STAsimply.

Further, the APs AP1 and AP2 are functional entities which provideconnection to the DS via a wireless medium for the STAs associatedtherewith. In principle, communication between the non-AP STAs is madevia the AP in the infrastructure BSS including the AP. However, when adirect link is set up between the non-AP STAs, direct communication canbe made between the non-AP STAs.

The AP may be called a convergence controller, a Base Station (BS), anode-B, a Base Transceiver System (BTS), a site controller, and so on.Also, an AP which supports ultrahigh speed data processing over 1 GHz ina multi-channel environment described below is called a VHT AP.

A plurality of infrastructure BSSs can be connected to each otherthrough a Distribution System (DS). The plurality of BSSs connectedthrough the DS are called an Extended Service Set (ESS). STAs includedin the ESS can communicate with one another, and non-AP STAs can movefrom one BSS to another BSS in the same ESS while communicating withoutany interception.

The DS is a mechanism for allowing one AP to communicate with anotherAP. Here, the AP can transmit a frame to STAs associated with the BSSwhich the AP manages, transfer a frame when any STA moves to anotherBSS, or transfer a frame to an external network such as a wired network.The DS may not be a network necessarily. As long as the DS can provide acertain distribution service defined in IEEE 802.11, there is nolimitation on a type of the DS. For example, the DS may be a wirelessnetwork such as a mesh network or a physical structure connecting theAPs to each other.

FIG. 2 is a diagram illustrating inter-frame spaces applied to a channelaccess mechanism in the WLAN system.

A channel access mechanism in accordance with IEEE 802.11 uses aDistributed Coordination Function (DCF) based on a Carrier SenseMultiple Access/Collision Avoidance (CSMA/CA).

The CSMA/CA protocol is intended to reduce a collision probability in apoint where a plurality of STAs accessing one medium may probablycollide with one another. For example, such collision probably occursdirectly after the medium ends an idle state. It is because many STAshave waited until the medium is available. In this situation, a randombackoff procedure is required to prevent medium contention conflict.

According to the CSMA/CA of the DCF, the STA performs the backoffprocedure before starting transmission. This period is a contentionwindow (CW) determined by the number of slot time. Therefore, the STAsselect a backoff timer before accessing a channel. After the backofftimer is expired, the STAs can access the channel. The backoff timer isarbitrarily selected from the CW. An initial value of the CW is set as aparameter CWmin, and a maximum value thereof does not exceed CWmax.

The backoff procedure reduces a collision probability when several STAsattempt to access the channel. After a certain time elapses, when theSTA senses an idle state of the channel or medium again, the STA countsdown, decrementing a backoff timer value at the time of pausing thebackoff procedure.

The CW can be represented as a positive integer having a value betweenCWmin and CWmax. Methods for setting a new CW value are divided bywhether frame transmission is successful or not.

When the STA does not receive acknowledgement regarding data frametransmission after accessing the channel, it judges that collision hasoccurred, doubles the CW, and performs the backoff procedure again.

In this manner, as the time elapses, the STA has a value smaller than arandom backoff time of another STA, acquires a high priority, andfinally uses the channel.

On the contrary, when the STA receives acknowledgement regarding dataframe transmission after accessing the channel, it resets the CW to beCWmin which is a minimum value, and performs another backoff procedurefor next frame transmission.

Inter-frame spaces defined in the DCF include a Short Inter-Frame Space(SIFS), a PCF Inter-Frame Space (PIFS) and a Distributed Inter-FrameSpace (DIFS). The inter-frame space means a minimum idle time setbetween transmitted frames. The STA judges that the medium is in an idlestate during this time interval.

The SIFS indicates a response time spent until a response frame istransmitted. That is, after a data frame is transmitted, its responseframe (or ack frame) is transmitted after the SIFS time.

The PIFS indicates an idle time to transmit a CF-Poll frame so that theAP can generate a contention free period. That is, after the PIFS timeelapses, if the channel is in an idle mode, the AP can transmit theCF-Poll frame.

The DIFS indicates an idle time to transmit a frame in a contentionperiod. After the DIFS time elapses, if the channel is in an idle mode,the STA can perform a backoff procedure to transmit a frame.

Meanwhile, IEEE 802.11e uses an Enhanced Distributed Channel Access(EDCA) technique to improve Quality of Service (QoS). According to theEDCA technique, a concept of an Access Category (AC) is defined toprovide differentiated services based on priorities. Each AC uses apredetermined contention parameter and an Arbitration Inter-Frame Space(AIFS[AC]) replacing the DIFS, and has differentiated CWmin[AC] andCWmax[AC].

Provided are four ACs, Access Category Voice (AC_VO), Access CategoryVideo (AC_VI), Access Category Best Effort (AC_BE) and Access CategoryBackground (AC_BK).

The STA has four transmission queues which are recognized as a QoSparameter and a virtual STA for determining priorities. A separate queueexists for every AC. As explained above, CWmin and CWmax have differentvalues for every AC. In addition, the AIFS has a different value forevery AC.

If backoff counter values of a plurality of ACs located in parallel inone STA reach ‘0’ at the same time, a scheduler provided in the STAprevents virtual collision.

In the respective frames, the AIFS[AC} is set to have different sizesaccording to priorities defined in the ACs. A frame of a top priorityhas an Inter-Frame Space (IFS) approximate to the DIFS, and a frame of alower priority has a longer IFS. As a result, a frame having a highpriority more possibly accesses the medium.

The CW also has different CWmin[AC] and CWmax[AC] according to thepriorities. A frame of a higher priority has smaller CWmin[AC] andCWmax[AC]. Therefore, even if collision occurs, the frame of the highpriority can reduce a waiting time for accessing the medium. It is ofadvantage when data of the transmission queue acquire a transmissionopportunity.

Briefly, the higher the priority is, the smaller CWmin, CWmax and AFISare set. It is thus possible to provide differentiated servicesaccording to the priorities for every AC.

FIG. 3 is a diagram illustrating a channel access method according toone embodiment of the present invention.

As set forth herein, the channel access method explained with referenceto FIG. 3 can be used in an idle band such as a TV white space.

When a competition-based channel access mechanism is used, a problemcommonly arises in scalability. Particularly, a problem may arise incoexistence as well as scalability in the idle band. In order to solvethe foregoing problems, there is a method using Request To Send(RTS)/Clear To Send (CTS), or a method of adjusting the number ofcompeting STAs by an AP. A method for solving or reducing such problemsvia channel access control will be suggested in the embodiment of thepresent invention.

Referring to FIG. 3, a transverse axis indicates time flow. Here, theaccess of STAs is not sequential necessarily as shown in FIG. 3, and thesequence of the STAs accessing a channel is not limited to the sequenceof FIG. 3. The sequence of the STAs shown in FIG. 3 is intended forconvenience of explanation, which is not relevant to the embodiment orright scope of the present invention.

FIG. 3 illustrates a channel access mechanism where a beacon intervalbetween a beacon frame 310 and a beacon frame 315 includes one or morecontention periods (CPs).

The entire time is divided into a Contention-Free Period (CFP) and aContention Period (CP). Wireless devices operate for the CP in the DCFand for the CFP in the PCF.

Here, an Access Point (AP) transmits a channel access control message tothe STAs, to thereby control channel access of the STAs. Forconvenience's sake, an early transmitted channel access control messageis referred to as a first channel access control message 320. The APtransmits the first channel access control message 320 containinginformation on the STA to be allowed to access the channel, therebydesignating the STA to be allowed to access the channel among theassociated STAs.

The contents of the first channel access control message 320 will bedescribed later in more detail with reference to FIG. 4.

In this chase, the AP broadcasts the first channel access controlmessage 320. There is no limitation on the STAs which can receive thefirst channel access control message 320. That is, the AP transmits thefirst channel access control message 320 in an omni-directional mode sothat all the STAs can hear it.

The STAs receiving the first channel access control message 320 from theAP confirm whether they are allowed to access the channel, respectively.For example, if the first channel access control message 320 containsMAC address information equivalent to an MAC address of a correspondingSTA, the STA can access the channel for a predetermined time.

Alternatively, a TIM element may be used to judge whether channel accessis allowed, which will be described later with reference to FIG. 4.

When the AP intends to change the STAs allowed to access the channel, itre-transmits a channel access control message. For convenience's sake, asecond transmitted channel access control message is referred to as asecond channel access control message 340. The second channel accesscontrol message 340 is also broadcasted to all the STAs. The AP can addinformation designating STAs other than the STAs allowed to access thechannel via the first channel access control message 320 to the secondchannel access control message 340.

The STAs which were allowed to access the channel via the first channelaccess control message 320, and receive the second channel accesscontrol message 340, but are disallowed to access the channel via thesecond channel access control message 340 pausing accessing the channel.

A channel access control management frame may be transmitted directlyafter the PIFS time so as to improve a priory.

As described above, the AP can adjust the number of the competingterminals in the DCF and EDCA through the channel access control message320 or 340. Consequently, the effect is same as when a beacon intervalis divided into several contention periods. The list and number of thecompeting terminals may be changed in each contention period.

FIG. 4 is a diagram illustrating the channel access control messagetransmitted by the channel access method explained with reference toFIG. 3.

The control message may follow a format of a channel access controlmanagement frame. The channel access control management frame maycontain information on a MAC address, an Associated ID (AID) and aTraffic Indication Map (TIM) of terminals allowed to access the channel.

That is, according to the foregoing embodiment, the AP broadcasts thechannel access control management frame.

A category field 410 indicates a frame category, i.e. a correspondingframe is a management frame for controlling channel access. In addition,an action field 420 indicates that the corresponding frame is intendedfor an operation of allowing or disallowing channel access of the STA.

A channel access control mode field 430 has one octet. For example, withrespect to allowing or disallowing channel access, if a field value ofthe channel access control mode field 430 is ‘1’, it means ‘allow’, andif the field value is ‘0’, it means ‘disallow’.

A conventional TIM element can be used to indicate whether the AP hasdata to be sent to the STAs operating in a power saving mode. In thiscase, the TIM element may be contained in a beacon frame. However,according to the embodiment of the present invention, a TIM field 440 iscontained in the channel access control management frame, and used toindicate whether the STAs receiving the channel access controlmanagement frame are allowed to access the channel through the DCF andthe EDCA.

Accordingly, the STAs receiving the channel access control managementframe confirm the TIM field 440 first, and then confirm whether a bit ofan AID field 450 has been set as ‘1’ for the corresponding STAs. If thebit of the AID field 450 is set as ‘1’, the STA starts contention-basedchannel access through the DCF or EDCA.

If not, e.g. if the TIM field 440 does not exist in the frame or the bitof the AID field 450 is set as ‘0’, the corresponding STAs pauseaccessing the channel.

The channel access control management frame may contain an Associated ID(AID) 450 and a MAC address 460 of the terminal allowed to access thechannel, instead of the TIM field 440.

That is, the STA judges whether it is allowed to access the channel bychecking the AID field 450 or the MAC address field 460. For example,when the same AID or MAC address information is contained in the channelaccess control management frame, or when a field value of acorresponding field of the received channel access control managementframe is set as an appointed value, the corresponding STA can access thechannel.

In the opposite case, the STA does not access the channel or pausesaccessing the channel.

The TIM field 440, the AID field 450 or the MAC address field 460 may beoptionally contained in the channel access control management frame. Ifsuch fields are contained in the channel access control managementframe, one or more of the TIM field 440, the AID field 450 and the MACaddress field 460 may be selectively contained therein.

When the TIM field 440, the AID field 450 and the MAC address field 460are contained in the channel access control management frame, thechannel access control mode field 430 can be used as informationindicating whether channel access is allowed, and the TIM field 440, theAID field 450 or the MAC address field 460 can be used as informationspecifying the STA allowed to access the channel.

For example, if the channel access control mode field 430 is ‘1’, theSTAs specified via the TIM field 440, the AID field 450 or the MACaddress field 460 are allowed to access the channel. On the contrary, ifthe channel access control mode field 430 is ‘0’, the terminalsspecified via the TIM field 440, the AID field 450 or the MAC addressfield 460 are not allowed to access the channel.

Meanwhile, a case where the TIM field 440, the AID field 450 and the MACaddress field 460 are not contained in the channel access controlmanagement frame will be explained later with reference to FIG. 6.

FIG. 5 is a diagram illustrating a channel access method according toanother embodiment of the present invention.

According to another embodiment of the present invention, an AP candesignate terminals to be allowed to access a channel among associatedterminals by dividing them in a spatial manner.

When a service area of the AP is divided into several sectors, a sectorin which STA1 531, STA2 533 and STA3 535 are located is referred to assector 1, and a sector in which STA4 551 and STA5 553 are located isreferred to as sector 2.

The AP transmits a first channel access control message 520 and a secondchannel access control message 525 to sector 1, and transmits a thirdchannel access control message 540 to sector 2. A method of transmittinga channel access control message to a specific sector can be used whenthe AP supports a directional antenna. That is, the AP supporting thedirectional antenna can transmit the channel access control messagetoward STAs in a specific sector.

In this situation, since the STAs are automatically specified by atransmission direction, the channel access control message notifies allthe STAs that channel access is allowed or disallowed.

First of all, when the AP intends to allow the STAs in sector 1 toaccess the channel, the AP transmits the first channel access controlmessage 520 to sector 1, i.e. STA1 531, STA2 533 and STA3 535.

In addition, when the AP intends to end a contention period and changean area allowed to access the channel, the AP transmits the secondchannel access control message 525 to sector 1. Here, as mentionedabove, sector 1 is a sector currently allowed to access the channel.

If the first channel access control message 520 is a message forallowing channel access, the second channel access control message 525is a message for disallowing or pausing channel access. The AP transmitsthe second channel access control message 525 to sector 1, to therebynotify that all the STAs in sector 1 are not allowed to access thechannel.

Next, the AP transmits the third channel access control message 540 tosector 2 which is a sector to be changed, i.e., a sector to be newlyallowed to access the channel. The third channel access control message540 contains the contents that all the STAs in the corresponding sectorare allowed to access the channel, like the first channel access controlmessage 520. However, while the first channel access control message 520is intended to allow the STAs in sector 1 to access the channel, thethird channel access control message 540 is intended to allow the STAsin sector 2 to access the channel. STA4 551 and STA5 553 are allowed toaccess the channel via the third channel access control message 540.

As the AP transmits the third channel access control message 540, acontention period for sector 2 starts. The STAs in sector 2 receivingthe third channel access control message 540 access the channel. Thischannel access procedure for every sector may be called a ‘sectoredchannel access mechanism.

FIG. 6 is a diagram illustrating the channel access control messagetransmitted by the channel access method explained with reference toFIG. 5.

Basic functions of a channel access control management frame shown inFIG. 6 are identical to those of the channel access control managementframe shown in FIG. 4. However, in the embodiment explained withreference to FIG. 5, the AP transmits the frame limitatively to the STAsto be allowed or disallowed to access the channel.

Accordingly, the channel access control management frame can containonly a category field 610, an action field 620 and a channel accesscontrol mode field 630. As described above, the category field 610indicates a frame category, i.e. a corresponding frame is a managementframe for controlling channel access, and the action field 620 indicatesthat the corresponding frame is intended for an operation of allowing ordisallowing channel access of the STA.

For example, the AP can transmit the frame merely to STAs located in acertain sector, using a directional antenna or the like. Therefore, inthis case, information for specifying STAs needs not to be contained inthe channel access control management frame. Accordingly, the channelaccess control management frame according to this embodiment may notcontain fields such as TIM, AID and MAC address.

When the TIM, AID and MAC address fields are not contained in thechannel access control management frame as in the embodiment shown inFIG. 6, whether all the STAs receiving the channel access controlmanagement frame are allowed to access the channel is determined by avalue of the channel access control mode field 630. For example, whenthe value of the channel access control mode field 630 is ‘1’, all theterminals are allowed to access the channel. On the contrary, when thevalue of the channel access control mode field 630 is ‘0’, all theterminals are not allowed to access the channel.

Moreover, the channel access control management frame may be transmittedto end channel access. The channel access control management frame isdifferent from the channel access control management frame shown in FIG.4 in that the action field 620 has two different values according towhether the frame is intended to allow channel access or endalready-started channel access.

For example, when the field value of the action field 620 is ‘1’, STAsin a certain sector are allowed to access the channel. Thereafter, whenthe channel access control management frame where the field value of theaction field 620 is set as ‘0’ is transmitted to the STAs in the sector,the STAs in the sector cannot attempt to access the channel any more.

Alternatively, the channel access control mode field 630 may havedifferent values to indicate whether the frame is intended to allowchannel access or end already-started channel access. For example, whenthe value of the channel access control mode field 630 is set as ‘1’,all the STAs receiving the corresponding channel access controlmanagement frame can access the channel, and when the value of thechannel access control mode field 630 is set as ‘0’, all the STAsreceiving the corresponding channel access control management framepause accessing the channel.

Here, the contents or values of each field are provided for an exemplarypurpose, and thus may be set differently for every embodiment.

FIG. 7 is a diagram illustrating a Traffic Indication Map (TIM) of thechannel access control message explained with reference to FIG. 4 or 6.The TIM element will be explained in detail with reference to FIG. 7.

As explained above, the TIM element contained in the TIM field indicateswhether an AP has data to be sent to STAs operating in a power savingmode.

The TIM may contain an element ID field 710, a length field 720, aDelivery Traffic Indication Message (DTIM) count field 730, a DTIMperiod field 740, a bitmap control field 750, a partial virtual bitmapfield 760, and so on.

The DTIM count field 730 indicates how many beacon frames appear,including the current frame, before a next DTIM. When a DTIM count valueis ‘0’, it means that the current TIM is a DTIM. Here, the DTIM countfield is one octet.

The DTIM period field 740 indicates how many beacon frames exist betweenbeacon interval continuing DTIMs. If all the TIMs are DTIMs, a value ofthe DTIM period field 740 is ‘1’. The DTIM period field 740 is oneoctet.

The bitmap control field 750 is also one octet. Bit 0 of the bitmapcontrol field 750 contains a traffic indicator bit associated withAssociation ID (AID) 0.

The partial virtual bitmap field 760 of the TIM indicates whether framesheading for associated STAs have been buffered.

For example, when the AP is buffering a frame for an STA having AID1, afirst bit of the partial virtual bitmap field 760 is expressed as ‘1’.

The DTIM is a special TIM element. A format of a DTIM element isidentical to a format of a TIM element. However, the DTIM informs that amulticast frame and a broadcast frame have been buffered in the AP.

If the AP buffers the multicast frame and the broadcast frame, a 0-thbit of the partial virtual bitmap field 760 can be expressed as ‘1’.Here, ‘0’ which is one of the values of the AID field is a reservedvalue for a multicast or broadcast session.

All the methods described herein can be performed by processors such asmicro-processors, controllers, micro-controllers, and ApplicationSpecific Integrated Circuits (ASICs) using softwares or program codescoded to perform the same, or a processor of the terminal shown in FIG.3. Design, development and realization of such codes are obvious tothose of ordinary skill in the art by the description of the presentinvention.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

1. A method of controlling channel access in a WLAN system, comprising:receiving a first channel access control message sent from an accesspoint to stations located in some area through a directional antenna ofthe access point; and attempting the channel access according to thefirst channel access control message.
 2. The method of claim 1, whereinthe first channel access control message contains information on acontention period in which the channel access is allowed.
 3. The methodof claim 1, comprising: receiving a second channel access controlmessage through the directional antenna of the access point; and endingthe channel access according to the second channel access controlmessage.
 4. The method of claim 3, wherein the first channel accesscontrol message and the second channel access control message contain achannel access control mode field, and a field value of the channelaccess control mode field contained in the first channel access controlmessage is different from a field value of the channel access controlmode field contained in the second channel access control message.
 5. Amethod of controlling channel access in a WLAN system, comprising:receiving, at a station, a first channel access control message from anaccess point; and attempting a channel access, if the station is one ofthe stations allowed to access the channel according to the firstchannel access control message.
 6. The method of claim 5, wherein thefirst channel access control message contains information on the stationallowed to access the channel.
 7. The method of claim 6, wherein theinformation on the station allowed to access the channel is MAC addressinformation of the station.
 8. The method of claim 6, wherein theinformation on the station allowed to access the channel is traffic IDinformation of the station.
 9. The method of claim 5, furthercomprising: receiving, at the station, a second channel access controlmessage from the access point; and pausing, at the station, accessingthe channel, when the station is included in stations disallowed toaccess the channel according to the second channel access controlmessage.
 10. The method of claim 9, wherein the first channel accesscontrol message contains information on the station allowed to accessthe channel.
 11. The method of claim 10, wherein the information on thestation allowed to access the channel is MAC address information of thestation.
 12. The method of claim 10, wherein the information on thestation allowed to access the channel is traffic ID information of thestation.